Orogen Hinterland Evolution: Testing Hypotheses using the Cretaceous to Eocene Stratigraphic Record in Eastern Nevada, USA

内华达中东部羊口组及相关单元的沉积记录了晚白垩世至晚始新世塞维尔腹地的重要地质历史。这项研究为各种模型提供了一个关键的测试，这些模型涉及塞维尔腹地随时间的运动作用，腹地过程如何与前陆相关，以及造山高原如何从缩短的顶点演化到伸展崩塌。特别是关于羊口组底部巨角砾岩沉积的时间和沉积背景的问题，直接关系到塞维尔腹地是否经历了与中地壳流动有关的伸展，还是表明上地壳在晚白垩世至古新世期间与中地壳有效分离的序外缩短。此外，本研究还验证了羊口组上覆的巨角砾岩沉积和主要角不整合的发育是否表明在始新世中晚始新世塞维尔腹地从缩短到伸展的重大转变。作为该项目的一部分，我们正在进行详细的地质测绘、地层测量和构造分析，以确定绵羊关组类型剖面内巨角砾岩矿床和同沉积断层的构造背景。新的氩氩定年法首次提供了羊口组的绝对年龄控制，将羊口组顶部的主要角不整合面和大角砾岩进行了划分，并允许构造事件的区域对比。新获得的化石资料也为相对定年提供了新的机会。我们目前正在整理来自内华达州中东部羊口组其他地区的地层测量、化石数据、地质年代学、物源和古传播数据，以重建该地区晚白垩世至始新世的地质历史和古地理。此外，我们与亚利桑那大学合作的铀铅碎屑锆石定年提供了重建区域无顶历史的机会，以确定新生代火山活动的开始时间，以及火山活动与塞维尔腹地构造之间的联系。同样，我们与斯坦福大学合作测量羊口组内湖泊碳酸盐的氧同位素比率，将有助于确定同位素变化与构造事件的关系。虽然我们的重点是在塞维尔造山系的一部分发生了什么，但我们的结果对其他地区的古代造山带以及现代造山系具有启示意义。对腹地高原构造和古高程史认识的提高，使我们能够与已确定的外部褶皱冲断带的演化进行更全面的比较。通过在国家和区域会议上发表报告以及通过最终出版地图和同行评议的科学期刊文章，正在与研究界分享与该项目有关的研究成果。研究结果将使构造学、构造地质学、沉积学、地层学、地质年代学、火成岩岩石学、古生物学和同位素地球化学等领域的工作者受益并感兴趣。该项目促进了博士生Peter Druschke和两名本科生的专业发展。尤其是Druschke，通过与一位沉积学家-地层学家、一位构造地质学家、一位稳定同位素地球化学家、两位地质年代学家和一位古生物学家的学习和互动，他正在接受盆地分析方面的特殊多学科培训，并为学术生涯中的竞争性研究做准备。此外，本科生将与Druschke和PI密切合作，并将从调查实地研究，接触各种先进的分析技术和数据分析的机会中受益匪浅。此外，参与该项目的学生和合作者将受益于多个学术机构之间的合作和交流。该项目得到了美国国家科学基金会构造和EPSCoR项目的资金支持。

The deposition of the Sheep Pass Formation and correlative units of east-central Nevada records a critical geologic history of the Sevier hinterland during Late Cretaceous to Late Eocene time. This study provides a critical test to a variety of models pertaining to the kinematic role of the Sevier hinterland through time, how hinterland processes relate to the foreland, and how orogenic plateaus may evolve from the culmination of shortening to extensional collapse. In particular, questions pertaining to the timing and depositional setting of megabreccia deposits at the base of the Sheep Pass Formation relate directly to whether the Sevier hinterland experienced extension related to mid-crustal flow, or out-of-sequence shortening indicating that the upper crust was effectively decoupled from the mid-crust during the latest Cretaceous to Paleocene. Additionally, this study tests whether megabreccia deposition and the development of major angular unconformities overlying the Sheep Pass Formation indicate a major shift from shortening to extension within the Sevier hinterland during the Middle to Late Eocene. As part of this project, we are conducting detailed geologic mapping, stratigraphic measurements, and structural analyses to determine the tectonic setting of megabreccia deposits and syndepositional faults within the Sheep Pass Formation type section. Our new Argon-Argon dating provides the first absolute age control for the Sheep Pass Formation, brackets major angular unconformities and megabreccia at the top of the Sheep Pass Formation, and allows for regional correlations of tectonic events. Newly obtained fossil data also provides new opportunities for relative dating. We are currently compiling stratigraphic measurements, fossil data, geochronology, provenance and paleodispersal data from additional localities within the Sheep Pass Formation throughout east-central Nevada to reconstruct the Late Cretaceous to Eocene geologic history and paleogeography of the region. Additionally, our Uranium-Lead detrital zircon dating in collaboration with the University of Arizona provides opportunities to reconstruct the regional unroofing history, to identify the timing of initiation of Cenozoic volcanism, and the links between volcanism and tectonics within the Sevier hinterland. Similarly, our collaboration with Stanford University in measuring the oxygen isotope ratios of lacustrine carbonates within the Sheep Pass Formation will help determine how isotopic shifts relate to tectonic events. Although our focus is on what happened in one part of the Sevier orogenic system, our results have implications for ancient orogens in other regions, as well as modern orogenic systems. Our improved understanding of the tectonic and paleoelevation history of the hinterland plateau allows us to make a more comprehensive comparison to the well-determined evolution of the external fold-thrust belt. Research findings related to this project are being shared with the research community through presentations at national and regional conferences and through the eventual publication of maps and peer reviewed scientific journal articles. Results will both benefit and be of interest to workers within the fields of tectonics, structural geology, sedimentology, stratigraphy, geochronology, igneous petrology, paleontology and isotopic geochemistry. This project facilitates the professional development of Peter Druschke, a Ph.D. student, and two undergraduate students. Druschke in particular is receiving exceptional multidisciplinary training of great breath in basin analysis, and preparation for competitive research in an academic career, through study and interaction with a sedimentologist-stratigrapher, a structural geologist, stable isotope geochemist, two geochronologists, and a paleontologist. The undergraduate students additionally will work closely with the Druschke and the PI's and will benefit immensely from opportunities for investigative field research, exposure to a variety of advanced analytical techniques, and data analysis. In addition, students and collaborators involved with this project will benefit from cooperation and exchange between multiple academic institutions. This project is being supported with funds from the NSF Tectonics and EPSCoR programs.